1. Field of the Invention
The present invention relates to an outboard motor including an internal combustion engine provided with an intake system for carrying air for combustion, namely, intake air, into a combustion chamber, an engine cover forming an engine compartment in which the internal combustion engine is placed, a top cover covering the engine cover from above, and an intake silencer.
2. Description of the Related Art
A known outboard motor disclosed in, for example JP 5-286490 A, JP 2006-151242 and U.S. Pat. No. 3,610,198 includes an internal combustion engine provided with an intake system for carrying intake air for combustion into a combustion chamber, an engine cover forming an engine compartment in which the internal combustion engine is placed, a top cover covering the engine cover from above. In this outboard motor, intake air taken into an air-intake space defined by the engine cover and the top cover is carried into the intake system disposed in the engine compartment.
When the intake system disposed within the engine compartment of the outboard motor opens into the air-intake space outside the engine compartment, cool intake air can be taken into the intake system, as compared with a case in which intake air is taken into the engine compartment and warmed by heat radiated from the engine in the engine compartment. Thus, the internal combustion engine can be charged at an improved charging efficiency and the output performance of the engine can be improved when such intake air at lowered temperature is taken into the intake system.
When the intake system opens into the air-intake space outside the engine compartment, the intake pulsation caused by the internal combustion engine is transmitted through the intake system to the air-intake space. Since the air-intake space is defined by the top cover and the engine cover, the engine cover is vibrated by the intake pulsation transmitted to the air-intake space to generate noise.
When an air-intake passage connecting an air-intake opening into the air-intake space to the intake system is formed in a labyrinthine shape to prevent water collected in the air-intake space from flowing into the intake system, intake resistance increases to deteriorate the output performance of the internal combustion engine.